Minor: convert to expression body is forbidden also for nested non-exhaustive whens

idea/testData/intentions/convertToExpressionBody/whenWhenNonExhaustive.kt

@@ -0,0 +1,18 @@
+// IS_APPLICABLE: false
+enum class AccessMode { READ, WRITE, RW }
+fun whenExpr(mode: Boolean, access: AccessMode) {
+    <caret>when (access) {
+        AccessMode.READ -> when (mode) {
+            true -> println("read")
+            false -> println("noread")
+        }
+        AccessMode.WRITE -> when (mode) {
+            true -> println("write")
+        }
+        AccessMode.RW -> when (mode) {
+            true -> println("both")
+            else -> println("no both")
+        }
+    }
+}
+fun println(s: String) {}